Legions of big predatory squid have gathered along the Northern California coast, where they are stranding themselves on Santa Cruz beaches by the hundreds in a mysterious frenzy of suicide.

The jumbo invertebrates, known as Humboldt squid, are far north of their normal habitat in the warmer waters of Baja California and along the west coast of South America.

Nobody knows why the aggressive, tentacled creatures moved north, but they have been showing up along the Santa Cruz and Monterey coasts with increasing frequency over the past decade, according to researchers.

"We're definitely a little baffled, but it's exciting because we've heard about these strandings for so long and now it's happening right here," said Hannah Rosen, a graduate student and researcher at the laboratory run by California's pre-eminent squid researcher, William Gilly, at Stanford University'sHopkins Marine Station.

"It's not typical for them to be in Monterey Bay. It is a fairly recent occurrence for them to be here at all," Rosen said. "We're hoping that since it has been happening pretty much in our backyard that we will be able to make some headway and find out what is going on."

The squid found on the beaches this past weekend were all between 2 and 3 feet long, dark red with large, bulging eyes and long tentacles extending outward from a small, toothy mouth.

The creatures, which can reach almost 5 feet in length and will eat almost anything smaller than they are, including their own species, have been lurking in Monterey Bay since summer. Dead squid were first reported in October, when about 100 washed up in Pacific Grove. Several hundred washed ashore over the weekend on beaches along a 12-mile stretch in Santa Cruz County, one of the largest local mass strandings that researchers can remember.

Last seen in 2010

Rosen said the voracious squid, known scientifically as Dosidicus gigas, were last seen in Monterey Bay in 2010. The squid in the bay have primarily been juvenile squid, she said, probably because the young need to feed in a bay until they are big enough to head south. The animals can live up to 2 years.

The presence of this particular species is not likely to make scuba divers happy. Humboldt squid, so named because they were first discovered in the Humboldt Current in South America, are also known in Spanish as diablo rojo, or red devil, because they have a reputation for aggression. The species, which are among the largest carnivorous squid in the world, often approach and sometimes even grab scuba divers.

"They are not afraid to approach divers and touch them. They'll grab you with their arms and try to figure out what you are," said Rosen, adding that the sea creatures display unusual intelligence. "It can make people a little uncomfortable. With a squid that big, it's nothing to scoff at, but there has been no documentation of them trying to eat a human. They are just curious."

Some scientists believe the squid are provoked by divers wearing reflective gear or holding flashing lights.

The ability of the animals to change color is another reason for the devilish reputation, Rosen said. In the presence of other squid, they will flash from white to red and back in a remarkable imitation of a strobe light.

The species are believed to have first ventured up to the Central Coast from the Sea of Cortez in 2000. Scientists with the Gilly lab believe they move north during El Niño weather conditions, which are in effect this year. It is believed that the water to the south becomes too warm for the squid. Climate change could be accelerating the warming process, which would explain why there have been more recent squid sightings.

Stranded newcomers

Researchers say strandings are common when squid invade a new area. The squid, which typically spend their days 1,000 to 2,000 feet deep and come up to feed at night, actually swim onto the beach and strand themselves.

Laboratory scientists don't know why, but they believe the squid spend so much time in the deep ocean that they never encounter land. The species is nearly impossible to keep in captivity because they are unused to barriers and boundaries and will continually run up against aquarium glass and kill themselves, Rosen said.

"Another theory is that there is a harmful red algae bloom, which can secrete a neurotoxin called domoic acid, and it could be affecting their central nervous system and causing them to become disoriented," Rosen said.

The laboratory is conducting tests to determine whether the dead squid were infected with neurotoxin.